The present invention relates to a process for the storage and conversion of energy by means of a cell having an anode compartment, a cathode compartment and an ion exchange membrane as the electrolyte and to an apparatus for carrying out the same.
Electrolytic cells of this type are known. By way of illustration, a cell having a special ion exchange membrane as the electrolyte is described in U.S. Pat. No. 4,175,165 (Adlhart). Other cells of this type are described in U.S. Pat. No. 3,779,8Il (Bushnell et al.), U.S. Pat. No. 3,297,484 (Niedrach) and U.S. Pat. No. 4,529,670.
All the cells described in these patents have an anode compartment, a cathode compartment and an ion exchange membrane as the electrolyte; however, they are of greatly varying design and construction.
The cells known from these patents are employed in particular as fuel cells. These are galvanic cells that continuously convert the chemical change in energy in a fuel oxidation reaction into electric energy, with a differentiation being made between "hot" and "cold" combustion.
In "hot" combustion in an oven, the electron transition from the fuel molecule (usually hydrogen), which serves as the electron donor, to the oxidation molecule (usually oxygen), which serves as the electron acceptor, is usually irreversible in the short circuit due to the direct contact of the two reactants, with heat being generated.
In "cold" combustion in a fuel cell, however, this electron transition is largely reversible at two separate points: at the negative electrode (anode), the fuel molecules strip the electrons with positive fuel ions being formed in the surrounding electrolyte, and at the cathode (positive electrode), the oxidant molecules picking up electrons form negative oxidant ions in the electrolyte. If the anode and the cathode are connected via an electric user, the oxidation reaction proceeds to the extent as the user consumes, e.g., "demands" current. In cold combustion in the fuel cell, most of the reaction heat becomes available in the user as high-quality electric energy. Fuel and oxidant ions migrate in the electrolyte and unite there to form a reaction product, by way of illustration in the case of hydrogen/oxygen to form water, with the electric circuit being closed.
As previously explained, the individual reactions in a fuel cell, by way of illustration in a hydrogen/oxygen fuel cell, are reversible so that they can run inversely.
If the mode of operation of a cell having an anode compartment, a cathode compartment and an ion exchange membrane as the electrolyte is reversed, the apparatus no longer operates as a fuel cell, but rather as an electrolyzer (hereinafter referred to as electrolytic cell) which generates the electrolytic gases hydrogen and oxygen against a certain overpressure.
For this reason, it has been numerously proposed to utilize a cell having an anode compartment, a cathode compartment and an ion exchange membrane as the electrolyte for the conversion and the storage of energy in order to store electric energy, in particular, over long periods of time, e.g. a year, in this manner. Long storage periods of this kind are, e.g., required for the economic exploitation of solar energy. The use of secondary elements for storing solar energy over a long period of time is not only very expensive, but also not very efficient due to the high degree of self-discharging over longer periods of time.
The attempts at developing a fuel cell that may be utilized for both the conversion and the storage of energy hitherto described in the literature (e.g., Hydrogen/Oxygen (Society of Plastics Engineers) electrochemical devices for Zero-G applications. Proceedings of the European Space Power Conference; Madrid, Spain, October 2-6, 1989) are always based on a bifunctional oxygen electrode and a bifunctional hydrogen electrode. In other words, during electrolytic operation at the one electrode, oxygen is formed and during fuel cell operation, oxygen is reduced. At the other electrode during electrolytic operation, hydrogen is formed, and during fuel cell, operation hydrogen is oxidized.
However, the following problems crop up in cells of this type having a cation exchange membrane as the electrolyte.
Due to the selection of the catalyst for the oxygen electrode, a loss in efficiency must be accepted as platinum, which is required for the reduction of oxygen, is not the best electrocatalyst for the formation of oxygen and, therefore, diminishes the effectivity of the electrolysis.
In order for the oxygen reduction to take place in the fuel cells with cation exchange membranes, moreover, in order to remove the reaction and transfer water, a hydrophobic layer, e.g. in the form of hydrophobic graphite paper, is required at the electrocatalyst. During electrolysis, however, this hydrophobic graphite paper dissolves due to the anodic oxidation resulting from the high anodic potential of the oxygen formation.
An object of the present invention is to provide a process for the conversion and the storage of energy that operates with an electrolytic cell which has an ion exchange membrane as the electrolyte that possesses a high degree of effectivity both in storing electric energy over a long period of time (e.g., a year) or short periods and at the same time permits converting energy.
Another object of the present invention is to provide a process that operates with a H.sub.2 /O.sub.2 /H.sub.2 O-system.
Furthermore, an object of the present invention is to provide an apparatus for carrying out the aforementioned process.
These and other objectives are achieved by the present invention which provides a process for the storage and the conversion of energy by a cell having an anode compartment, a cathode compartment and an ion exchange membrane as the electrolyte. The cell is operated as a fuel cell for converting energy, and as an electrolytic cell for storing energy. In the anode compartment, a bifunctional oxidation electrode is used; while in the cathode compartment, a bifunctional reduction electrode is used.
In the process of the present invention, a fuel cell having an ion exchange membrane as the electrolyte is employed. The electrode arrangement in the anode compartment is a bifunctional oxidation electrode so that during electrolytic operation, by way of illustration, oxygen is formed and during fuel cell operation, by way of illustration, hydrogen is oxidized. A bifunctional reduction electrode is also utilized in the cathode compartment so that (e.g.) hydrogen is formed during electrolytic operation and oxygen is reduced during fuel cell operation.
Therefore, contrary to the state of the art, oxygen is not formed at an electrode, e.g. during electrolytic operation and oxygen is not reduced during fuel cell operation. Rather, when the mode of operation is switched from electrolysis to fuel reaction or vice versa, the type of gas switches between oxygen and hydrogen in the anode compartment and the cathode compartment. In other words, at one electrode, oxygen is formed during electrolytic operation and hydrogen is oxidized during fuel cell operation; while at the other electrode, hydrogen is formed during electrolytic operation and oxygen is reduced during fuel cell operation.
Therefore, an element of the present invention is that the process utilizes such bifunctional oxidation and reduction electrodes. Employing these bifunctional electrodes permits attaining improved storage effectivity due to the ability to select a more suitable catalyst for both reactions of a bifunctional electrode. Thus, e.g., rhodium oxide or iridium oxide, which is a much better catalyst for oxygen formation than platinum, may be employed as the oxidation electrode. On the other hand, there appears to be no negative effects using rhodium oxide or iridium oxide as the electrocatalyst for hydrogen oxidation in comparison with platinum. Moreover, the hydrophobic graphite paper required for oxygen reduction is not destroyed by anodic oxidation. In this manner, the use of such bifunctional oxidation and reduction electrodes permits avoiding the two major difficulties in constructing an electrochemical cell having a cation exchange membrane as the electrolyte and possibly alternating use as a hydro-electrolyzer and a H.sub.2 /O.sub.2 fuel cell.
Advantageously, a cell of this type having bifunctional oxidation, or reduction, electrodes described above is operated in such a way that during electrolytic operation water is conducted through the anode compartment and the resulting oxygen and hydrogen is stored in the corresponding storage means.
Subsequently, the cell is operated as a fuel cell as required. The hydrogen and oxygen therefor is drawn from a storage means, with the hydrogen being conducted through the anode compartment and the oxygen through the cathode compartment.
The respective electrolysis cycles, or fuel cycles, may be repeated as often as desired. The duration of the cycles depends on the respective requirements and the layout of the cell. In the area of exploitation of regenerable energy, it is quite possible to store up to a year and then commence the fuel cell cycle. On the other hand, cycles ranging from minutes to hours can also be conducted for respective needs.
A feature of the present invention is that the hydrogen, or the oxygen, is not initially generated by electrolysis in the first step of the process, but rather that the hydrogen and the oxygen are drawn from an external gas storage means, e.g. natural gas deposits. The hydrogen/oxygen is conducted to the cell and the cell is subsequently operated as a fuel cell, i.e. hydrogen is conducted through the anode compartment and oxygen through the cathode compartment. Thereupon, the cell is operated as the electrolytic cell, with the hydrogen being conducted through the anode compartment as previously described above and the resulting oxygen and hydrogen being stored. Thereupon, the fuel cell and the electrolytic cycles can again follow.
Advantageously during the electrolytic cycle, this distilled water is conducted cyclically from the water container through the anode compartment. It is provided that, if need be, water from an external source can be conducted into the water container, which may be the case if there is a loss of water due to evaporation.
In another advantageous embodiment, it is provided that during the electrolytic cycle the oxygen formed in the anode compartment is carried along by the water circulating in the cycle, separated from the water in a gas separator, which is connected thereafter, and subsequently conducted to the oxygen storage means.
In a further advantageous embodiment, the oxygen is conducted via a gas dryer, which is connected before the oxygen storage means. In this way, it is ensured that the oxygen storage means contains only dried gas so that it can again be immediately conducted into the next cycle.
Moreover, another feature of the present invention is that the transfer water, i.e. water which is transported through the ion exchange membrane by the protons evolved during the electrolytic cycle, is removed from the system by a water separator, which is connected after the cell. The resulting hydrogen is then conducted into a hydrogen storage means. In the case of hydrogen, an advantageous embodiment of the present invention provides that the hydrogen is conducted into the hydrogen storage means via a gas dryer in which is connected before the storage means.
In accordance with the present invention, the operational gases are conducted cyclically through the corresponding cathode, or anode, compartments during the fuel cell process. Subsequently, the oxygen is conducted cyclically from the oxygen storage means out through the cathode compartment and hydrogen is conducted cyclically from the hydrogen storage means through the anode compartment. In an advantageous embodiment the oxygen, or hydrogen, is conducted through a wetting agent/condenser prior to being introduced into the cell.
Another feature of the present invention is that the cell is scavenged with a stream of inert gas, e.g. nitrogen, when the mode of operation is switched, i.e., between the individual electrolytic, respectively fuel cell, cycles. This ensures that no residual gases from a previous cycle are in the cell.
The cell employed in the process of the present invention has specially designed bifunctional oxidation, or reduction, electrodes and an ion exchange membrane as the electrolyte, a hydrophobic structure, a material distribution system and current collectors.
The bifunctional electrodes are constructed in such a manner that they include a catalyst layer which is applied to the membrane and/or to a porous carrier as a stable unit between the material distribution system and the membrane, or the hydrophobic structure and the membrane and/or the material distribution system, or the hydrophobic structure. The amount of catalyst may lie between 0.1 and 10 mg/cm.sup.2. For both sides, the catalyst may be platinum, rhodium, ruthenium, palladium, osmium, rhenium and/or alloys thereof and/or oxides. Preferably, rhodium and iridium are utilized.
As hydrophobic structures, carriers such as graphite, carbon tissue, metal structures of titanium, niobium, tantalum or zirconium may be employed which have become hydrophobic due to treatment with perflourinated synthetic material.
The other material distribution systems utilized in accordance with the present invention are, by way of example, porous sintered bodies made of titanium, niobium, tantalum or zirconium having a mean pore size from 1-200 .mu.m and a thickness of 0.5-2 mm. Fine wire mesh made of titanium, niobium, tantalum, zirconium or platinum may also be employed.
The used collectors are provided with coarse structures made of titanium, niobium, tantalum or zirconium which possess the mechanical stability for pressing the cells together, make the water and gas transport to the material distribution structures possible and permit drawing the current.
Advantageously, the cells have an ion exchange membrane as the electrolyte. Best suited for this has proven to be a cation exchange membrane, as e.g. the commercially available NAFION.RTM. 117, semipermeable membrane of poly(perflouroalkylen)-sulfonsaure manufactured by DuPont de Nemours.
The apparatus of the present invention has a cell which is operated in accordance with the present invention in such a manner that during electrolysis oxygen is formed at one electrode and during fuel cell operation hydrogen is oxidized. At the other electrode, hydrogen is formed during electrolysis and oxygen is reduced during fuel cell operation.
In order to be able to supply this bifunctional oxidation, or reduction electrode with the corresponding gasses, or with H.sub.2 O, the cell is constructed in such a manner in accordance with the present invention that both the anode compartment and the cathode compartment have suitable inlets, or outlets. Especially advantageous is if the inlets, or outlets, are connected to control devices, e.g., multiway valves. In this way, it can be ensured that the gas can be switched by simply turning such a multiway valve if there is a corresponding gasline for conducting gas to and from the water reservoir and the gas storage means.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.